A hearing aid system typically comprises an input transducer, an audio input signal processing unit and an output transducer. The input transducer can be a microphone and is configured to receive sound from an ambient environment and to convert the received acoustic signal into an electric audio input signal. In the audio input signal processing unit, the electric audio input signal is processed by applying a specific processing mode. After processing the electric audio input signal, the audio input signal processing unit provides an electric audio output signal which is received by the output transducer. The output transducer, subsequently, provides an output signal that is based on the electric audio output signal and that can be perceived as sound by a user of the hearing aid system.
In everyday life, hearing situations in view of ambient sound that a person faces can vary e.g. between a silent ambient environment, a normal noise level and a very noisy or loud ambient environment.
To provide that all important information such as speech intelligibility that are contained in the electric audio output signal are presented to a user of a hearing aid system in an appropriate way, an audio input signal processing unit typically comprises different processing modes for processing an electric audio input signal. Different processing modes typically implement a processing algorithm that at least in part is different to processing algorithms used in other processing modes. E.g., if the received sound contains a high noise level, the respective electric audio input signal can be processed using a different processing mode than a processing mode used for processing an electric audio input signal that is based on a received sound signal having a low noise level. Processing algorithms may use parameter values that can be set in a hearing aid setting procedure. Hearing aid settings are parameter value sets that determine the function of at least one processing algorithm in a processing mode.
Typically, hearing aid settings can be adjusted based on an estimated signal-to-noise ratio. If a high noise level is detected, a processing mode comprising a noise reduction algorithm can be applied for processing the electric audio input signal. Alternatively, or in combination with e.g. noise reduction, hearing aid settings can be adjusted towards turning on microphone directionality. If the detected noise level is low, the complexity of the applied processing mode can be reduced such that it can be expected that the power consumption is reduced.
However, it is known that the ability to understand speech or to filter speech information contained in a noisy signal varies considerably from listener to listener. This not only depends on a degree of a person's hearing impairment but also depends on the individual cognitive ability. Furthermore, speech comprehension can even vary with the time of a day as speech comprehension can be influenced by the level of fatigue/sleepiness or the current motivation of understanding speech. Consequently, applying different processing modes according to predefined threshold values, e.g. according to predefined threshold values of the signal-to-noise ratio, can sometimes be highly insufficient.
In recent years, researchers have focused on investigating the relationship between speech recognition and a person's cognitive load (see e.g. E. B. Petersen et al., Front Psychol. 2015; 6:177). The term “cognitive load” refers to the total amount of mental effort being used in a person's working memory. Due to the fact that a person's cognitive load typically increases with increasing difficulty of the listening task, the cognitive load has gained an increasing interest for applications in hearing aid devices. Consequently, there exist a variety of attempts of estimating a person's cognitive load and using said estimate for hearing aid applications.
In US 2016/008076 A1 a method of operating a hearing instrument is presented that inter alia comprises adapting the processing of an input signal in dependence of the estimate of the present cognitive load.
Different methods for estimating a person's current cognitive load have been suggested, such as by using electrodes that are placed on the surface of an in-the-ear part of a hearing aid device. The electrodes pick-up a low voltage signal from a user's brain to record the electric activity of the brain. The resulting electroencephalogram (EEG) displays measured voltage fluctuations over time.
EP 2950555 A1 discloses a hearing aid capable of monitoring a user's hearing ability over time. The described hearing aid comprises at least one electrode that is adapted to pick up a low voltage signal from a user's brain.
EP 2744224 A1 describes a system of operating a hearing system. The system comprises one or more electrodes for measuring the brain wave signals of a user wearing the hearing instrument.